Tomáš Vystavěl

Guided Assembly of Gold Colloidal Nanoparticles on Silicon Substrates Prepatterned by Charged Particle Beams

Colloidal gold nanoparticles represent technological building blocks which are easy to fabricate while keeping full control of their shape and dimensions. Here, we report on a simple two-step maskless process to assemble gold nanoparticles from a water colloidal solution at specific sites of a silicon surface. First, the silicon substrate covered by native oxide is exposed to a charged particle beam (ions or electrons) and then immersed in a HF-modified solution of colloidal nanoparticles. The irradiation of the native oxide layer by a low-fluence charged particle beam causes changes in the type of surface-terminating groups, while the large fluences induce even more profound modification of surface composition. Hence, by a proper selection of the initial substrate termination, solution pH, and beam fluence, either positive or negative deposition of the colloidal nanoparticles can be achieved.

Low energy focused ion beam milling of silicon and germanium nanostructures

In this paper focused ion beam milling of very shallow nanostructures in silicon and germanium by low energy Ga( + ) ions is studied with respect to ion beam and scanning parameters. It has been found that, using low energy ions, many scanning artefacts can be avoided and, additionally, some physical effects (e.g. redeposition and ion channelling) are significantly suppressed. The structures milled with low energy ions suffer less subsurface ion beam damage (amorphization, formation of voids) and are thus more suitable for selected applications in nanotechnology.

In-situ observation of 〈110〉 oriented Ge nanowire growth and associated collector droplet behavior

Using in-situ microscopy, we show that germanium nanowires can be grown by a vapor-liquid-solid process in 〈110〉 directions both on Ge(100) and Ge(111) substrates if very low supersaturation in the collector droplet is ensured. This can be provided if thermal evaporation is utilized. Such a behavior is also in agreement with earlier chemical vapor deposition experiments, where 〈110〉 oriented wires were obtained for very small wire diameters only. Our conclusions are supported by in-situ observations of nanowire kinking towards 〈111〉 direction occurring more frequently at higher evaporation rates.

Real-Time Observation of Collector Droplet Oscillations during Growth of Straight Nanowires

A liquid droplet sitting on top of a pillar is crucially important for semiconductor nanowire growth via a vapor-liquid-solid (VLS) mechanism. For the growth of long and straight nanowires, it has been assumed so far that the droplet is pinned to the nanowire top and any instability in the droplet position leads to nanowire kinking. Here, using real-time in situ scanning electron microscopy during germanium nanowire growth, we show that the increase or decrease in the droplet wetting angle and subsequent droplet unpinning from the growth interface may also result in the growth of straight nanowires. Because our argumentation is based on terms and parameters common for VLS-grown nanowires, such as the geometry of the droplet and the growth interface, these conclusions are likely to be relevant to other nanowire systems.

Controlled faceting in 〈110〉 germanium nanowire growth by switching between vapor-liquid-solid and vapor-solid-solid growth

We show that the hexagonal cross-section of germanium nanowires grown in the 〈110〉 direction by physical vapor deposition is a consequence of minimization of surface energy of the collector droplet. If the droplet is lost or solidified, two {001} sidewall facets are quickly overgrown and the nanowire exhibits a rhomboidal cross-section. This process can be controlled by switching between the liquid and solid state of the droplet, enabling the growth of nanowires with segments having different cross-sections. These experiments are supported by in-situ microscopic observations and theoretical model.

The Synergic Effect of Atomic Hydrogen Adsorption and Catalyst Spreading on Ge Nanowire Growth Orientation and Kinking

Hydride precursors are commonly used for semiconductor nanowire growth from the vapor phase and hydrogen is quite often used as a carrier gas. Here, we used in situ scanning electron microscopy and spatially resolved Auger spectroscopy to reveal the essential role of atomic hydrogen in determining the growth direction of Ge nanowires with an Au catalyst. With hydrogen passivating nanowire sidewalls the formation of inclined facets is suppressed, which stabilizes the growth in the ⟨111⟩ direction. By contrast, without hydrogen gold diffuses out of the catalyst and decorates the nanowire sidewalls, which strongly affects the surface free energy of the system and results in the ⟨110⟩ oriented growth. The experiments with intentional nanowire kinking reveal the existence of an energetic barrier, which originates from the kinetic force needed to drive the droplet out of its optimum configuration on top of a nanowire. Our results stress the role of the catalyst material and surface chemistry in determining the nanowire growth direction and provide additional insights into a kinking mechanism, thus allowing to inhibit or to intentionally initiate spontaneous kinking.

Fatigue Crack Initiation in Crystalline Materials – Experimental Evidence and Models

Recent observations relevant to the early stages of the fatigue damage of crystalline materials are reviewed. Experimental evidence on the localization of the cyclic plastic strain and on the surface relief formation in cyclic loading of 316L austenitic stainless steel is presented. The focused ion beam is used for exposing three-dimensional evidence of persistent slip markings (PSMs). PSMs consist of extrusions and parallel or alternating intrusions which develop during cyclic loading. Monte Carlo simulations of vacancy generation within persistent slip band (PSB) and their migration to the matrix where they annihilate on the edge dislocations are used to simulate the growth of extrusions and intrusions. The results of the simulations are compared with experimental data and discussed in terms vacancy models of fatigue crack initiation.

New approaches to in-situ heating in FIB/SEM systems

Over last decades significant effort has been made on in-situ heating experiments inside SEM and FIB/SEM chambers. Traditional way is to use low vacuum environment in the entire chamber. Although this valuable approach brings various undeniable advantages, new state of the art experiments coincide with new requirements, such as rapid changes in temperature, high-vacuum operation to maximize experiment cleanliness, ultra-high resolution SEM imaging and on top of it adaptable geometry in order to investigate sample’s crystallography and composition changes using EBSD and EDS detectors. In this contribution we introduce an integration of two new modules fulfilling these requirements by allowing in-situ heating in FIB/SEM systems under high vacuum conditions. Moreover, heating in high vacuum combined with injection of selected gases was also proven capable of providing sample surface oxidation [1] or reduction (Figure 1), [2].

MEMS-based Heating Element for in-situ Dynamical Experiments on FIB/SEM Systems

In-situ observation of microstructural evolution of solids such as recrystallization, grain growth and phase changes in SEM is important for various fields of material science and industry research. This technique requires reliable discrimination of differently oriented crystal phases combined with useful spatial and temporal resolution and with fast and precise control of specimen temperature. While the requirements on spatial and temporal resolution are satisfied by current SEMs with resolution below 1 nm and 100 Hz frame rate, existing heating holders for bulk samples only allow for heating rates up to 300oC per minute (5oC/s). Long ramping time, which is required during heating experiments done using these devices, may cause unwanted sample changes (e.g. oxidation or recrystallization) before the temperature range of interest is reached. Thermal radiation of massive heating holders decreases quality of material contrast imaging as the commonly used detectors of backscattered electrons become saturated by thermally emitted photons. MEMS-based heating holder [1], [2] in combination with in-situ site specific sample preparation using a FIB/SEM system brings significant improvement in instrumentation for in-situ heating experiments inside the SEM chamber.

Expanding Capabilities of Low-kV STEM Imaging and Transmission Electron Diffraction in FIB/SEM Systems

High resolution low-kV STEM imaging is getting more and more attention in the materials research and semiconductor industry, as well as in life sciences research [1]. This has been driven by the need to work with thinner TEM lamella, and with samples containing low-Z and beam sensitive materials. Decreasing electron energies is often favorable from a radiation damage point of view, moreover it improves scattering contrast. In this contribution we focus on STEM imaging extended by diffraction analysis, by integration of pixelated detectors into the FIB/SEM platform.